R is for REFLEX!
This is from my 500mm reflex lens...reflex, in this context, is just another name for a mirror lens. I've had this old Minolta lens for quite a few years. Sony continued the manufacture of this lens after the Minolta take over but they discontinued it's production earlier this year...BUMMER...many manufacturers build reflex lenses but the Minolta/Sony was in a class of it's own. It was the only mirror lens to feature automatic focus. That was a pretty impressive bit of engineering!